The present invention concerns a road surfacing with reduced rolling noise emission and a method for its manufacture.
A considerable environmental problem in today's society is noise emission from roads and streets. At lower speeds and high acceleration, in most cases noise from the power unit dominates at the receiver point as compared to the rolling noise. At speeds exceeding 50 km/h, however, rolling noise generated by the tire/road contact will dominate. This means that even along streets and roads in built-up areas, a considerable part of the noise emission is caused by the tire/road noise. In such areas it would not be particularly helpful to further reduce noise emission from the power units. The fact that tire/road rolling noise aleady dominates the total noise at 50 km/h contributes to a certain understandable disinclination of car manufacturers to further reduce noise emission from the power units. Thus, a reduction of the tire/road rolling noise would solve acute noise problems for streets and highways where speeds generally exceed 50 km/h and also would reduce traffic noise even at lower speeds, since reduced tire noise levels will increase the motivation to also reduce the noise emission of the power unit.
Tire/road noise is generated by the contact between the tire and the road surface. The major part of the sound radiation occurs from the tire close to the contact patch. For tires with more powerful tread patterns this takes place from the trailing portion of the contact patch. At lower frequencies (below about 800 Hz), tire/road noise is dominated by direct radiation of the tire carcass vibrations. At higher frequencies, air-resonances between tire and road surface will probably substantially influence noise radiation. This means that a considerable noise reduction could be achieved if such air-resonances are not excited to a greater extent. To achieve this it is very important that air pressure neutralization between tread pattern cavities occur.
Methods known up to now for tire/road noise reduction are:
1. Measures on tread pattern, i.e. distributing the length of tread blocks in direction of rotation so that tonal components are spread around the mean frequency, thereby reducing tonal peaks; PA1 2. Changing the rubber compound so that higher compliance is obtained; and, PA1 3. Influencing the road surface texture so that an optimum texture depth with regard to tire noise is obtained.
Further developing these constructional principles could give 2-4 dB(A) in additional noise reduction. Laboratory studies have revealed that the most probable cause of the high frequency noise production from tires (most important for the perceived noise impression) is that tire tread block oscillations cause air movements. If the positive and negative pressures, respectively, which are created in the tire grooves could be equalized, the noise would decrease.